1. Field of the Invention
The present invention relates to an optical scanning system for facsimile transmitters, receivers, etc. which carries out optical spot scanning at a high speed by using a rotatable reflecting mirror.
2. Description of the Prior Art
In case of a facsimile receiving devices in general which performs optical spot scanning, scanning of a photosensitive surface (scanning surface) 4 is carried out, as shown in FIG. 1, by a light beam from light source 1, which is modulated by respective image information elements in turn, by using a pinhole 2 and a scanning element 3 comprising a lens system or the like. As the scanning surface in this case is two dimensional, scanning of a flat surface is carried out by combining two linear scanning directions. Out of two dimensions, scanning for one dimension is generally carried out by moving the photosensitive surface along a straight line or by moving the scanning lens along a straight line in the scanning direction by keeping the photosensitive surface fixed. For scanning for the remaining one dimension out of two dimensions, a facsimile head utilizing optical fiber, for example as shown in FIG. 2, is used in most cases. That is, the light beam from the light source 11 is led in turn, by rotating, onto a circular portion 13a which is one end face of an optical fiber 13 by means of a rotary member 12. The light beam led onto said circular portion 13a is led to the straight portion 13b, which is the other end portion of the optical fiber 13, and scans the photosensitive surface 14.
FIG. 3 shows another known linear scanning method in which the above-mentioned facsimile head utilizing optical fiber is not used. That is, between an objective 33 and the photosensitive surface 34, a reflecting mirror 35 is rotatably arranged. This type of scanning means to rotate a reflecting mirror has various advantages compared with the scanning means by linear movement. That is, as movement for operation is smoother, high-speed scanning is possible and, moreover, the scanning device can be manufactured easily. Especially, when a rotary polyhedric mirror is used instead of simply rotating a reflecting mirror and, moreover, when a powerful light source such as a LASER beam is employed, scanning at still higher speed becomes possible.
However, said scanning method utilizing a rotatable reflecting mirror has a disadvantage that the distance between the lens system and the spot to be scanned is not constant. That is, as shown in FIG. 3, the conjugate position for the pinhole 32 in relation to the lens system 33 is on a circular arc 36 which is drawn by using a point near the intersecting point of the surface of the reflecting mirror 35 and optical axis 33a of the lens system 33 as the center and using the distance B between the center 34a of the photosensitive surface 34 and reflecting mirror 35 as a radius. Therefore, when reference symbol A represents an angle between a line from the center of said reflecting mirror to the center 34a of the photosensitive surface 34 and a line from the center of said reflecting mirror to the outermost portion 34b of the photosensitive surface 34, a distance C between said circular arc 36 and photosensitive surface 34 is expressed by a formula C= B (sec A-1). Therefore, when size of the photosensitive surface 34 is about B4 size (approximately 26.times. 36 mm) and the distance B is about 400 mm, the angle A becomes about 18.degree.. Therefore, the distance C becomes about 20 mm and out-of-focus phenomenon occurs. Consequently, favourable scanning cannot be attained.